Natural gas fuels are relatively environmentally friendly for use in vehicles, and hence there is support by environmental groups and governments for the use of natural gas fuels in vehicle applications. Natural gas based fuels are commonly found in three forms: Compressed Natural Gas (CNG), Liquefied Natural Gas (LNG) and a derivative of natural gas called Liquefied Petroleum Gas (LPG).
Natural gas fuelled vehicles have impressive environmental credentials as they generally emit very low levels of SO2 (sulphur dioxide), soot and other particulate matter. Compared to gasoline and diesel powered vehicles, CO2 (carbon dioxide) emissions of natural gas fuelled vehicles are often low due to a more favourable carbon-hydrogen ratio found in natural gas. Natural gas vehicles come in a variety of forms, from small cars to buses and increasingly to trucks in a variety of sizes. Natural gas fuels also provide engines with a longer service life and lower maintenance costs. Further, CNG is the least expensive alternative fuel when comparing equal amounts of fuel energy. Still further, natural gas fuels can be combined with other fuels, such as diesel, to provide similar benefits mentioned above.
A key factor limiting the use of natural gas in vehicles is the storage of the natural gas fuel. In the case of CNG and LNG, the fuel tanks are generally expensive, large and cumbersome relative to tanks required for conventional liquid fuels having equivalent energy content. In addition, the relative lack of wide availability of CNG and LNG refuelling facilities, and the cost of LNG, add further limitations on the use of natural gas as a motor vehicle fuel. Further, in the case of LNG, the cost and complexity of producing LNG and issues associated with storing a cryogenic liquid on a vehicle further limit the widespread adoption of this fuel.
While LNG has had some success as a liquid fuel replacement in some regions of the world, the lack of availability of LNG and its high cost means that in many regions of the world it is not a feasible alternative fuel. In the case of CNG, it also has had some success as a liquid fuel replacement but almost exclusively in spark ignition engines utilising low pressure carburetted port injection induction technology. This application is popular in government bus fleets around the world where the cleaner burning natural fuel is used in a spark ignition engine fitted in place of a conventional diesel engine.
Some of the above issues are also mitigated when using LPG, and this fuel is widely used in high mileage motor cars such as taxis. However, cost versus benefit comparisons are often not favourable in the case of private motor cars. Issues associated with the size and shape of the fuel tank, the cost variability of LPG and the sometimes limited supply mean that LPG also has significant disadvantages that limit its widespread adoption. In summary, unless there is massive investment in a network of LNG plants around major transport hubs, CNG is the only feasible form of natural gas that is likely to be widely utilised in the near future.
However, some technical problems still limit the efficiency of CNG fuel systems. For example, the pressure to which composite CNG cylinders can be filled at a typical CNG re-fuelling station is limited because the heat of compression can cause overheating of cylinders being filled. This has typically meant that a nominal 250 bar at 21 degrees Celsius (settled temperature) is the limit for composite CNG cylinder design, and has become the standard adopted in many parts of the world including the US.
In the US, codes typically allow for filling to an overpressure of 1.25 times the pressure rating of the CNG cylinder provided it would subsequently settle to a nominal 250 bar if cooled to 21 deg. C. The code also identifies in-cylinder heating as having the potential to cause transient temperature excursions exceeding cylinder design parameters, and these high temperatures also cause higher internal cylinder pressures such that fills of between 70% and 80% of cylinder “name plate” ratings are often all that can be achieved. This has a significant detrimental impact on the range of CNG vehicles, and also on consumers who often have difficulty understanding the variability of a CNG cylinder fill and the impacts on vehicle range.
Also, the variability and inability to fully fill CNG cylinders has a major impact on the use of CNG cylinders in bulk gas transport, where poor CNG cylinder filling has significant commercial impact on the cost of gas delivered.
For example, in Europe, the relevant codes limit the maximum pressure in composite CNG cylinders during re-fuelling to 260 barg to ensure maximum design temperatures are not exceeded. These limitations meant that the currently available composite cylinders designed for 350 barg operating pressure and above could not be utilised in conventional CNG re-fuelling systems. Thus the opportunity to utilise smaller CNG cylinders, or to achieve increases in vehicle range, or improved commercial outcomes for gas transport, using the same size fuel cylinders, can not be realised.
A further problem with current systems for fast refuelling of large CNG vessels, such as used in buses and trucks, is that the size and weight of the refuelling connection makes them difficult to handle and problematic relative to the smaller connectors used commonly for filling cars.
International Patent Application Publication, WO 2008/074075, titled “A COMPRESSED GAS TRANSFER SYSTEM”, disclosed for the first time a liquid backpressure system that enables the complete filling of on-vehicle CNG fuel tanks at full pressures. However, with this system the delivery of liquid into and out of CNG cylinders limits the application of the technology, and can slow transfer rates due to limitations in the liquid handling.
There is therefore a need for an improved system and method for refuelling compressed gas pressure vessels.